Cryogenic coolers (cryocoolers) are used to lower the temperature of the conductive components of superconducting devices to the temperature range where superconductivity occurs, at which temperature resistance drops to near zero, and conductors of relatively small cross sectional area can conduct very large amounts of current. In a conventional utility transformer for multiple-kilowatt industrial or residential power distribution, for example, fifteen tons or more of copper may be needed to allow the needed current to flow, and temperature equalization may be achieved by immersing the transformer in more tons of highly refined petroleum distillate (mineral) oil with properties resembling those of automobile motor oil, in some cases cooled with external radiators equipped with electric fans to couple the heat to the atmosphere. A superconducting transformer for the same utility load, by contrast, may have conductors weighing a ton or less, may occupy a fraction of the physical space, and may use no mineral oil.
Known superconductor materials may be categorized by their properties. Some are only marginally superconducting, and have such properties that large current flows cause their superconductivity to be lost. Others are superconducting only within a fraction of a degree of absolute zero (zero kelvins, or −273.15 degrees Celsius), requiring significant effort to maintain the working temperature against external environmental temperatures. Still others are highly frangible, so that they tend to crack under quite mild stresses, which can introduce resistive boundaries between superconducting segments and can cause destructive hot spots when such superconductors are used for high currents and variable loads. At this time, a superconductor that can carry industrial amperage at a temperature of 20 kelvins is viewed as a high-temperature superconductor. High-temperature superconductors that carry still higher currents at still higher temperatures may achieve commercial success in the future.
A representative electrical superconducting device may function economically when surrounded by an insulated container through which cooling media and wires for electrical power transmission may pass but through which heat travels with comparatively low efficiency. A container employing a thermal boundary region that is at least in part evacuated may be commonly referred to, and is herein, as a Dewar.
The output of a cryocooler using liquid nitrogen is capable of providing a heat sink that can be adapted to provide an effective thermal barrier between a Dewar's outer envelope and an inner, gaseous helium-based system, which can in turn maintain the temperatures needed for superconducting utility transformers. For such a system, the liquid nitrogen cryocooler can include a high-temperature, pressurized, gaseous segment, operating at a temperature high enough to couple its waste heat to another heat exchanger with useful efficiency; a low-temperature, high-pressure liquid segment, transporting phase changed nitrogen back to the heat sink; a low-temperature, low-pressure gaseous segment, in which heat of vaporization for the nitrogen is accepted from the Dewar, and a compressor segment in which the heat-bearing gaseous nitrogen is raised in temperature by pressurization.
A cryocooler working with helium as a thermal exchange medium may evolve its waste heat within a rather broad general range of room temperature, such as at temperatures in the vicinity of 200–400 K. Such a cryocooler can, for example, successfully and safely discharge its waste heat into a surrounding volume of air. Operating such a device in a full, worldwide outdoor climate can demand significant performance from scroll compressors and other cryocooler technology elements.
A helium cryocooler may employ substantially the same thermal cycle as the nitrogen cryocooler described above, with the exception that typical helium cryocoolers of current technology do not employ phase changes in the cycle, a simplification that allows the process to operate at a higher and thus less technically challenging range of temperatures. Heat coupling between a superconducting transformer and the helium of a cryocooler may take place in a further insulated chamber inside the Dewar, in which gaseous helium floods the transformer to capture the waste heat from the superconductor power management process. Heat coupling may also take place with the helium cryocooler functioning as a heat sink for the nitrogen cryocooler's heat exchanger as described above.
Accordingly, there is a need in the art for a rugged, long-life helium compressor/heat exchanger system to be housed adequately for all-weather operation, which compressor/heat exchanger can directly service large-scale, high-power superconductors, can mate with a nitrogen system and can provide effective discharge of waste heat to an uncontrolled environment.